1. Field of the Invention
The present invention relates to semiconductor devices and methods of fabricating the same. More particularly, the present invention relates to analog capacitors and methods of fabricating the same.
2. Description of the Related Art
An analog capacitor is one element of a device typically used in an analog-to-digital converter (ADC), a radio frequency (RF) device, a switching capacitor filter, a complementary metal oxide semiconductor (CMOS) image sensor (CIS), etc. An analog capacitor operates based on the fact that the quantity of charge stored in the electrodes on both ends of a dielectric layer varies corresponding to a change of an applied voltage.
When a voltage V is applied to a capacitor, an amount of charge Q stored in the electrodes of both ends of a dielectric layer can be given by Equation (1):Q=C×V  (1)where C is a capacitance of the capacitor.
If the capacitance C has a constant value, the amount of charge Q is linearly related to the voltage V. The amount of charge stored in the capacitor is constant when the voltage V is fixed. Accordingly, the voltage can be divided into intervals over an operating voltage range of a capacitor, and the amount of charge corresponding to each voltage interval can serve as one bit.
In order to obtain a high number of bits in an analog capacitor, the difference of the amount of charge corresponding to each voltage interval should be large and constant. For this purpose, the dielectric layer used in an analog capacitor needs a low voltage coefficient of capacitance (VCC), i.e., change in capacitance per change in voltage.
The capacitance of the dielectric layer generally depends on the voltage. That is, the capacitance C (V) is a function of an applied voltage V and may be fitted to a quadratic function, which is given by Equation (2):C(V)=C(0)×(a×V2+b×V+1)  (2)where C(0) is a capacitance of a capacitor with no applied voltage, “a” is a quadratic coefficient of VCC, and “b” is a linear coefficient of VCC. Accordingly, in order to have a low VCC, “a” and “b” should be close to zero.
In a conventional capacitor of a polysilicon/insulator/polysilicon (PIP) structure or a metal/insulator/polysilicon (MIP) structure, a depletion layer forms on a polysilicon electrode during operation. This depletion layer substantially increases the thickness of the dielectric layer, which reduces the capacitance of the capacitor and deteriorates the VCC properties. In order to prevent these problems, a metal/insulator/metal (MIM) structure is employed as the analog capacitor.
However, VCC properties of a MIM analog capacitor are deteriorated due to oxidation of an electrode. Several subsequent processes are performed after the lower electrode of the capacitor has been formed during which the surface of the lower electrode of the capacitor may be oxidized. In particular, as oxide series materials with a high dielectric constant replace a silicon nitride layer as the dielectric layer for high integration of the semiconductor device, the surface of the lower electrode may undergo oxidation during the process of forming the dielectric layer on the lower electrode. To prevent this, the surface of the lower electrode is being subjected to rapid thermal nitridation (RTN) or to nitrogen plasma processing. Even using these processes, however, it is difficult to fully prevent the surface of the lower electrode from being oxidized. In particular, suppression of the oxidation of the lower electrode becomes more difficult when a subsequent oxygen curing process is performed to enhance the quality of the capacitor dielectric layer.
When the lower electrode of the analog capacitor is oxidized as described above, the resistance of the oxidized portion increases and the depletion layer may form in the oxidized portion of the lower electrode during operation of the analog capacitor. This depletion layer increases the VCC value, particularly the linear coefficient b of Equation (2). In connection therewith, a capacitor and method of fabricating the same in which a laminated layer of dielectric layers with different electrical properties is used to enhance voltage efficiency properties of the capacitor has been proposed.